There is so much right with the idea of a Sherlock Holmes double genderswap, right? Sherlock Holmes's misogyny makes much more sense as Shirley Holmes's bluestocking/early feminist impulse; Shirley Holmes's skill for disguise comes from needing access to places women can't go.
Perhaps Shirley shares quarters with her dearest friend Johanna Watson, who, as a source of supplemental income, writes semi-fictionalized accounts of their adventures solving mysteries around London--with their names changed a bit, of course, and their genders switched because it would be scandalous otherwise. But perhaps Johanna, in the course of one of their mysteries, decided to accept Mr. Morstan's proposal; Shirley, of course, feels betrayed.
One of the things I love about the movie is that it's the male version of my favorite and least favorite story: "We Were Best Friends But Then She Got Interested In Boys And I Got Left Behind". I can't tell you how much I hate this story, and yet how tremendously compelling it is? And bringing it back to femslash/girl-BFFs makes it much more poignant for me. (Actually, the femslash version is even more depressing: We Were In Love But Then She Wanted To Get Married. To a Man." Blegh.)
It's hard to get across the sense of "Watson" without the mustache, haha, sorry. I gave Johanna fake sideburns using ringlets at least. (Note that Shirley is wearing the matching skirt to Johanna's jacket!)